This invention relates to gimbal antennas, and in particular, to eliminating the keyhole problem associated with azimuth-elevation gimbal antennas at their zenith position which requires extremely high angular acceleration motion of the azimuth motor during tracking a satellite near its zenith position.
Gimbal antennas are used for transmitting and receiving electrical signals to and from satellite vehicles. One type of gimbal antenna is known as the X-Y gimbal antennae. Such a gimbal antenna has the ability to rotate about the X and Y axes which are orthogonal to each other but not necessarily coplanar. Such X-Y gimbal antennas have a typically large sweep volume and, thus, are typically large in size for a given antenna aperture. Despite the disadvantage of the size, the X-Y gimbal antenna is rather common primarily because it does not have the keyhole problem near its zenith position.
Another type of gimbal is known as the azimuth-elevation gimbal antenna. Such a gimbal antenna is advantageous because it typically has a substantially smaller sweep volume than a corresponding X-Y gimbal antenna thereby allowing for an overall smaller antenna structure. An azimuth-elevation gimbal antenna is an antenna that is capable of rotating in two directions. The first rotational direction is in an azimuth direction which involves rotation of the antenna structure in a turntable motion in order to track the azimuth angle of a satellite vehicle. The second rotational direction is in the elevation direction which occurs by rotating the structure according to an elevation angle of a satellite. However, the keyhole problem with azimuth-elevation gimbal antennas occurs when the antenna is tracking a movable object, such as a satellite vehicle, near its zenith position, which is basically when the satellite vehicle is directly overhead. In this case, the azimuth motor hardly turns until the satellite approaches the zenith position and then the motor turns nearly 180 degrees within a short period as the satellite crosses the zenith position. FIG. 11 better illustrates the keyhole problem as the satellite is tracked from position 1 to the zenith position 2 and then to position 3 in the satellite motion plane. An azimuth-elevation gimbal antenna will successfully track when the satellite motion plane is exactly the same plane as the satellite-tracking plane. However; when the satellite motion plane and satellite tracking planes are slightly off co-planar, the keyhole problem is experienced whereby the azimuth motor must perform with extremely high rotational velocity. This rapid rotational motion of the antenna causes substantial problems in the acceleration of the gimbal antenna and could even cause its destruction.
Accordingly, a need exists for an improved method and apparatus for alleviating the keyhole problem associated with azimuth-elevation gimbal antennas thereby providing a suitable antenna system for tracking satellite vehicles while having a substantially smaller sweep volume and overall size than a corresponding X-Y gimbal antenna.